I. Field of the Invention
This invention relates in general to supported catalysts useful for the polymerization of olefins, especially alpha-olefins. More particularly, the invention relates to polymer supported catalysts of this kind, to methods of producing such catalysts and to the polymer supports themselves.
II. Description of the Prior Art
Polyolefins, especially polyethylene and its copolymers, are important commodity plastics with annual world production of over a billion pounds. Most polyolefins are produced by the well known Ziegler-Natta polymerization reaction which involves the use of a catalyst composed of a compound of a transition metal belonging to Group IV to VI of the Mendeleev Periodic Table (e.g. titanium, zirconium, vanadium or chromium) and an organometallic compound of a metal belonging to Group I to III of the Mendeleev Periodic Table (e.g. lithium, magnesium or aluminum).
While unsupported Ziegler-Natta catalysts may be used for polymerizations of this type, supported catalysts are now generally preferred for polyolefin production because they exhibit much higher activities than non-supported catalysts and therefore provide the advantage that the traditional de-ashing step to reduce titanium residue levels in the polymer products can be eliminated. Most of the materials used as supports for the catalysts are inorganic compounds, the most common being magnesium chloride as disclosed, for example, in U.S. Pat. Nos. 4,329,253 and 4,452,914.
Although the use of these highly active magnesium chloride-supported catalysts has resulted in a substantial reduction of titanium residue levels in polymer products, the contents of magnesium and chloride contributed by the supports themselves are not negligible and can have unfavorable effects on polymer processing and quality. For example, excessive chloride contents can cause corrosion problems in polymer processing equipment and excessive magnesium contents can cause moisture adsorption and water carryover in film fabrication.
It would therefore be advantageous to develop a support which could enhance the activity of Ziegler-Natta catalysts without introducing into the polymer products excessive levels of metal ions, halides or other inorganic impurities.
It has been suggested that the use of polymers as supports for the Ziegler-Natta catalysts could overcome these problems, for example in U.S. Pat. Nos. 4,268,418, 4,289,651, 4,329,255, 4,397,764, 4,477,639, 4,623,707, 4,632,912 and 5,051,484. However, it is fair to say that the development of polymer supported catalysts has not been very successful because the activities of such polymer-supported catalysts are usually not high enough for commercial use when compared to magnesium chloride-supported catalysts and because the supported catalysts are generally not easy to prepare.
Accordingly, there is a need for polymer supports for such catalysts that can result in higher catalytic activities that remain stable over suitable lengths of time.